The assemblies or orthogonal aggregates of particles within the cell membrane of astrocytes increase in number at the periphery of a cold lesion of the cerebral cortex in 9 day-old rats within 30 minutes. By 4 to 6 hours following the lesion, the number of assemblies is 4 to 5 times greater than that of resting astrocytes. Hypercapnia also leads to a marked and rapid augmentation. Within 30 minutes, there is a four-fold rise in assembly number. Weak acids, such as acetic and proprionic, when added to the culture medium, result in a decrease of about 10 to 3-fold, respectively, in the assembly number in astrocytes maintained in vitro for 14 days. Lactic acid did not appear to cause a significant change in the number of assemblies, but did cause them to aggregate. Etching of the uncleared astrocyte surface exposed to the lectin, concanavalin A, did not result in capping of surface particles, a finding which implies that the assemblies may not have an extrinsic, carbohydrate component.